TA benefited from a large, professional design staff, no matter how dated the material. Among other things, this allowed for such perks as professional sound effects, a professional soundtrack (which Spring, regrettably, does not support, but which's worth listening to:
http://gh.ffshrine.org/soundtracks/802), a large bulk of units and weapon types, and a creative vision of what the game was going to be from someone who had time to think about these things instead of trying to execute his crazy visions.
Creating a TA based mod can be as easy as opening a .sdz with 7zip and modifying the unit and weapon .fbis in notepad. Before you get started with any of that, though, let alone modeling and scripting for yourself, I'd recommend getting a solid idea of what exactly it is that you want to create, how it will play, and keep in mind some of the reasons that TA was a game worth modding to begin with.
At the very least, you'll want to consider things like:
Unit roles (by default Raiders, Main Line, All Terrain, Artillery, Information, Transport, Precision Support [Snipers/Bombers], Long-Distance Ballistic and Explosive, etc.).
Game speed (determined by unit movement speeds, build times, and resource income relative to spending).
Value of information (decided by movement speeds, weapon ranges, weapon AoEs, unit sizes, map sizes, LoS distances and radar/whatever distances).
Scale (decided by movement speeds, weapon ranges, unit sizes, map sizes and number of units).
Victory paths (Ground/Sea Assault, Ground/Sea Bombardment, Air Assault, Air Bombardment, Long-Range Artillery/Explosives, Special Forces [Cloaking, Jamming, Information and Micro]/whatever else you can think of).
Themes (point of the faction(s) doing what they do [you can be original and introduce a storyline into the loading screens], general style(s) of the army(ies), etc.).
Alternatively, you could be like most other modders, briefly consider a theme, and then just start throwing in units haphazardly based upon a general knowledge of the bare minimum that's required to make a game coherent, but no particular consideration or planning. Give a million monkeys a million typewrites and over a million years maybe they'll end up with Shakespeare. Free love and all that hippy Arm crap.... 0o